Add support of iOS aps content-available option

There is a new option is aps since iOS7.
You can check that in "Table 3-1  Keys and values of the aps dictionary" here :
https://developer.apple.com/library/ios/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/Chapters/ApplePushService.html
Merge pull request #56 from stephanecollot/patch-1

Add support of iOS aps content-available option